The term "chemical pulping" applies to the process of treating comminuted cellulosic fibrous material, for example, hardwood or softwood chips, with an aqueous solution of chemicals which dissolve the non-cellulose components of the material, and some of the cellulose components, to produce a slurry of cellulose fibers that can be used to produce cellulose paper products. The commercially significant chemical pulping process in the late twentieth century is the alkaline process, a process more commonly referred to as the "kraft" process. In the kraft process, the active chemicals with which the wood is treated are sodium hydroxide [NaOH] and sodium sulfide [Na.sub.2 S]. The aqueous solution of sodium hydroxide and sodium sulfide is referred to as "kraft white liquor".
Kraft pulping is typically performed at a temperature of over 100.degree. C., and the process is typically performed under superatmospheric pressure, preferably 5-10 bar, in a sealed pressure-resistant vessel known in the art as a digester. Typically, the cellulose material is sequentially raised to this treatment temperature and pressure, and cooking chemical is introduced to the material, in a series of steps that take place in what is known in the art as the "feed system".
In the case of a continuous digester in which material is continuously introduced at one end and discharged at the other, the feed system typically comprises or consists of several vessels for heating the material, raising its pressure, and introducing cooking liquid. For instance, continuous cooking feed systems typically include some form of chip bin into which the comminuted cellulosic fibrous material, referred to hereafter as "wood chips" (the most common form), are first introduced. This chip bin typically includes some form of isolation device at its inlet to prevent the escape of gases from the bin. The bin may also include an exhaust outlet for releasing the gases that may accumulate in the bin. Typically, treatment of the chips begins in the chip bin when the chips are exposed to high temperature steam. The steam begins the heating process, but, more importantly, the steam displaces the air in the chips so that the air content of chips is minimized. This removal of air and other gases from the chips promotes the "sinking" of the chips during subsequent aqueous treatment.
After steaming in the chip bin, the de-aerated chips are discharged from the chip bin by some form of metering device, for example, a Chip Meter sold by Ahlstrom Machinery Inc., of Glens Falls, N.Y. or a metering screw or any other form of conventional metering device. After discharge from the chip bin and metering device, the pressure of the chip mass is increased from approximately atmospheric pressure to a pressure of about 18 psi. This is typically achieved by a pressure isolation device, for example, a Low Pressure Feeder [LPF] as sold by Ahlstrom Machinery. The LPF is a device having a rotating star-type rotor within a stationary housing having an inlet and an outlet. Typically, as the rotor turns in the housing, chips drop through the inlet into the pockets of the rotor. As the rotor turns toward the outlet, the chips are exposed to a higher pressure and the chips fall through the outlet of the LPF to further treatment below. The clearance between the tines of the rotor and the inside surface of the housing is closely toleranced so that the higher pressure typically below the LPF does not escape to the area of lower, atmospheric pressure above and around the LPF.
The LPF typically includes some form of steam purge to purge the rotor cavities of chips during and after the chips are discharged from the outlet of the feeder. This purge usually comprises or consists of low-pressure steam introduced to a port in the housing of the feeder. The LPF also typically includes some form of exhaust gas relief port to release any gases that may accumulate in the feeder such that these typically pressurized gases are not introduced to the inlet of the feeder where they can interfere with the flow of chips into the feeder or interfere with the flow of chips through the metering device or chip bin above.
In conventional feed systems, the LPF discharges chips to the pressurized atmosphere of another treatment vessel. Conventionally, this vessel typically performs a further treatment of the chips with steam under a pressure of about 18 psi. This conventional pressurized steaming typically removes any further air that may be present and also increases the temperature of the chips to about 120.degree. C. prior to being immersed in cooking liquor. One preferred treatment vessel for performing this pressurized steam treatment is a Steaming Vessel as sold by Ahlstrom Machinery. The Steaming Vessel is most often a horizontally-oriented vessel having a cylindrical housing and horizontal screw conveyor. Steam is added to the housing through one or more ports typically located on the bottom of the housing. The source of this steam is typically flashed spent cooking liquor. That is, hot cooking liquor removed from the cooking process in the digester is expanded under controlled conditions by exposing the liquor to a pressure lower than its boiling point. In addition to generating steam from the flashed liquor, other volatile, typically malodorous, gases are also generated in the flashing process, such as hydrogen sulfide [H.sub.2 S], methyl mercaptan [CH.sub.3 SH], dimethyl sulfide [CH.sub.3 SCH.sub.3 ], and dimethyl disulfide [CH.sub.3 SSCH.sub.3 ], as well as other often malodorous gases. These gases, which are referred to collectively as Total Reduced Sulfur gases or TRS gases, are typically also introduced to the chips in the pressurized steaming process, typically in a Steaming Vessel.
Gases are also introduced to the Steaming Vessel from the outlet of the vessel which typically discharges to a vertical conduit or chute leading to a transfer device. For example, the outlet of the Steaming Vessel may discharge chips to a conduit leading to a star-type feeding device, for example, a High Pressure Feeder (HPF) sold by Ahlstrom Machinery, or to a slurry-type pump, for example, a LO-LEVEL.RTM. pump also sold by Ahlstrom Machinery. The conduits leading to these devices typically contain liquids containing sulfur compounds which also contribute TRS gases to the Steaming Vessel. Thus, the vessel below the LPF typically contains pressurized gases containing TRS compounds.
As a result, the outlet of the LPF typically is exposed to pressurized gases containing TRS compounds. These gases, if left unchecked, can be carried by the rotation of the LPF to the inlet of the LPF and released to the metering device and chip bin above. In addition, as discussed above, some LPF devices also include an exhaust port for discharging any accumulated gases from the LPF housing. Again, these TRS gases can typically be re-introduced upstream, for example, in the chip bin, and collected in the chip bin gas relief conduit. In conventional systems, this gas relief is directed to the Non-Condensable Gas (or NCG) collection system for destruction or re-use.
However, some pulp mills, typically older pulp mills, either do not have an NCG collection system or have an NCG collection system of limited capacity. Therefore, in such mills, it is undesirable to vent the TRS-laden gas streams in and around the LPF to the chip bin or to NCG treatment. In such systems, it is more desirable to re-introduce the TRS-laden streams to the feed system in a manner and form that does not allow the gases to escape to the atmosphere or be introduced to the NCG system. The present invention addresses this problem by removing the TRS-laden gases from the feed system and reintroducing these gases at a location downstream from where they were removed so that little or no TRS-laden gases are released to the atmosphere or must be treated or destroyed.
The broadest embodiment of this invention comprises or consists of a method and apparatus for minimizing the release of malodorous, TRS-containing gases from a pulp mill having a digester system and a feed system which feeds material to the digester system, wherein the method consists of or comprises the following steps: (a) introducing comminuted cellulosic fibrous material to the feed system; (b) exposing the material in the feed system to a pressurized gas containing TRS compounds, the gas having a first pressure; (c) removing the gas from the feed system at a first location; (d) pressurizing the gas and re-introducing the gas at a point downstream of said first location; and (e) discharging the material from the feed system and passing the material to the digester system for further treatment.
The digester system may be one or more continuous or batch digesters. The feed system typically includes one or more steam treating vessels, such as a Chip Bin or Streaming Vessel; one or more pressure isolation devices, such as a Low-pressure Feeder or High-pressure Feeder; and material transfer vessels, such as a Chip Chute or Chip Tube, and steps (b) and c) are practiced in one or more of these devices. The pressurization of step (d) is typically practiced using a thermocompressor, eductor, ejector, vacuum pump, compressor, or like device. Step (d) may be practiced by introducing the pressurized gas to any downstream location that can economically accommodate the introduction of a gas stream without interfering with the intended operation of the feed system or digester system. For example, the pressurized gas of step (d) may be introduced to the feed system, specifically to Steaming Vessel, Chip Tube, or Chip Chute; or the pressurized gas may be introduced to the digester system, specifically to a flash tank, condenser, or digester vessel, for example, to the top of a steam-phase digester vessel.
Another embodiment of this invention comprises or consists of a method and apparatus for capturing and re-introducing malodorous, TRS-containing, gases from a comminuted cellulosic fibrous material feed system without allowing the gases to escape to the environment. In the preferred embodiment, the method comprises transferring comminuted cellulosic fibrous material in a digester feed system having an isolation device followed by a treatment vessel containing malodorous gases, wherein the method consists of or comprises the following steps: (a) introducing comminuted cellulosic fibrous material at a first pressure to the inlet of a pressure isolation device; (b) transferring the material to the outlet of the device at a second pressure, higher than the first pressure; (c) discharging the material to the treatment vessel; (d) discharging malodorous gases that enter the isolation device from the isolation device; and (e) pressurizing at least some of the malodorous gases discharged from the isolation device to increase the pressure thereof (e.g. at least by 2 psig); and (f) re-introducing the pressurized malodorous gases to the cellulosic material flow (e.g. feed system or vessels) downstream of the pressure isolation device.
This disclosure also relates to a method and apparatus for minimizing the release of malodorous, TRS-containing, gases from a comminuted cellulosic fibrous material feed system having a pressure isolation device having an inlet and outlet and a treatment vessel connected to the outlet. The method comprises or consists of: (a) introducing comminuted cellulosic fibrous material at a first pressure to the inlet of a pressure isolation device; (b) transferring the material to the outlet of the device at a second pressure, higher than the first pressure; (c) discharging the material to the treatment vessel; (d) discharging the malodorous gases that enter the isolation device; and (e) introducing steam to the outlet of the isolation device to minimize or prevent the passage of malodorous gases from the treatment vessel through the housing of the isolation device.
There is provided a method of minimizing the release of malodorous TRS-containing gases from a comminuted cellulosic fibrous material feed system having a pressure isolation device with an inlet and outlet, and a treatment vessel connected to the outlet, the method comprising: (a) Introducing comminuted cellulosic fibrous material at a first pressure to the inlet of the pressure isolation device. (b) Transferring the material to the outlet of the pressure isolation device at a second pressure, higher than the first pressure. (c) Discharging the material from the pressure isolation device to the treatment vessel. (d) Introducing steam to the pressure isolation device to minimize or prevent the passage of malodorous gases into the treatment vessel through the pressure isolation device. And, (e) discharging malodorous gases from the pressure isolation device substantially independently of the discharge of comminuted cellulosic fibrous material therethrough.
The invention also consists of or comprises a method of minimizing the release of malodorous TRS-containing gases from a comminuted cellulosic fibrous material feed system, said method comprising: (a) Providing comminuted cellulosic fibrous material at a first pressure in the feed system and ultimately discharging the comminuted cellulosic material from the feed system. (b) Discharging malodorous gases from the feed system at a gas discharge point, substantially independently of the discharge of comminuted cellulosic fibrous material therefrom. (c) Pressurizing at least some of the malodorous gas discharged in (b) to increase the pressure thereof. And, (d) re-introducing the pressurized gas from (c) into the flow of comminuted cellulosic material downstream of the gas discharge point.
Typically (d) is practiced by reintroducing the malodorous gases into a treatment vessel connected to the outlet of the feed system, e.g. in a horizontal steaming vessel. The method may also further comprise (e) steaming the material in the horizontal steaming vessel, and discharging steamed material from the horizontal steaming vessel from a bottom portion thereof; (f) discharging malodorous gases from a top portion of the horizontal steaming vessel adjacent the bottom portion thereof from which the material is discharged; and (g) treating or disposing of the gases from (f) in an NCG system. Also the method may further comprise (h) directing or diverting the flow of gases from (b) to at least one of: (i) a chip bin operatively connected to the inlet of the pressure isolation device, (ii) atmosphere; and (iii) a pressurizing device which pressurizes the gases. For example (h) may be practiced by manual actuation causing a plurality of valves to be moved which control the passage of gas through conduits connected to the chip bin, to atmosphere, and to the pressurizing device.
Preferably (d) is practiced to increase the pressure of the gases to between about 11-31 psig, and at least one psig higher than the pressure in the vessel into which the gases are introduced, for example, the treatment vessel, or to at least increase the pressure by at least 2 psig.
According to another aspect of the present invention there is provided a method of minimizing the release of malodorous TRS-containing gases from a comminuted cellulosic fibrous material feed system having a pressure isolation device with an inlet and outlet, and a treatment vessel connected to the outlet, the method comprising: (a) Introducing comminuted cellulosic fibrous material at a first pressure to the inlet of the pressure isolation device. (b) Transferring the material to the outlet of the pressure isolation device at a second pressure, higher than the first pressure. (c) Discharging the material from the pressure isolation device to the treatment vessel. (d) Discharging malodorous gases from the pressure isolation device substantially independently of the discharge of comminuted cellulosic fibrous material therethrough. (e) Pressurizing at least some of the malodorous gases discharged in (d). And, (f) re-introducing the pressurized malodorous gases downstream of the pressure isolation device. Step (f) is preferably practiced by introducing the malodorous gases to the treatment vessel, but may be practiced by introducing the gases to any vessel downstream of the pressure isolation device. The method may further comprise screening the gases passing out of the pressure isolation device during (d) to substantially prevent the passage of chips, pins, or fines out of the pressure isolation device with the malodorous gases.
According to another aspect of the present invention a feed system for a digester (either a continuous digester or a plurality of batch digesters) in a pulp mill is provided. The feed system preferably comprises: A pressure isolation device having an inlet into which comminuted cellulosic fibrous material is fed at a first pressure, and an outlet from which the material is discharged at a second pressure, greater than the first pressure. A superatmospheric pressure treatment vessel having a material inlet connected to the outlet of the pressure isolation device, and a material outlet. A gas discharge outlet from the pressure isolation device separate and distinct from the material discharge outlet. A conduit connected to the gas discharge outlet. And a pressurizing device, which pressurizes gases, connected to the conduit. Also if desired there may be a screen at the gas discharge outlet for screening chips, pins and fines out of gas being discharged through the outlet.
The feed system may further comprise a pressurized fluid introduction port in the pressure isolation device, the port remote from the gas discharge outlet and closer to the material discharge outlet of the pressure isolation device than is the gas discharge outlet, and a screen at the gas discharge outlet for screening chips, pins and fines out of gas being discharged through the outlet. Preferably the pressurizing device comprises a thermocompressor or an eductor, connected to a source of steam providing a source of pressurizing fluid therefor. Typically a discharge of steam and pressurized gases from the thermocompressor or eductor is fed to the superatmospheric pressure treatment vessel at a point downstream of the pressure isolation device. For example the superatmospheric pressure treatment vessel comprises a horizontal steaming vessel; and the feed system further comprises a gas outlet from the horizontal steaming vessel operatively connected to an NCG system, the gas outlet downstream of the point at which the discharge of steam and pressurized gases is connected to the horizontal steaming vessel.
A plurality of conduits may be operatively connected to the gas discharge outlet and a manually or automatically operated valve controller provided to control the valves in the plurality of conduits. One of the conduits may lead to a chip bin operatively connected to the inlet of the pressure isolation device (e.g. through a chip meter), another conduit may lead to the atmosphere (e.g. a standpipe), the third conduit may lead to a pressurizing device which pressurizes gases.
It is the primary object of the present invention to provide an effective system and method for handling exhaust gases so as to minimize the potential for pollution from those exhaust gases. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.